Polishing stainless steel



ZSZ-l United States Patent POLISHING STAINLESS STEEL Selden C. Hayes and Joseph T. Lomakin, Baltimore, Md., assignors to Armco Steel Corporation, a corporation of Ohio No Drawing. Application April 6, 1950, Serial No. 154,419

11 Claims. (Cl. 41-42) This invention relates to stainless steel and more especilally to a method for brightening the surface of the stee An object of our invention is the provision of a simple, practical and reliable method for polishing st l and producing polished articles and products 0 the stee Another object of the present invention is that of providing a method for simultaneously pickling and brightening stainless steel to give a bright surface substantially free of pits, blemishes, scars and the like.

A further object of our invention is the provision of a direct and highly eflective method for treating stainless steel to alter the surface condition thereof, which method is easy to practice and afiords satisfactory working conditions in use.

Other objects of our invention in part will be obvious and in part pointed out more fully hereinafter.

The invention accordingly consists in the combination of elements, composition of ingredients, and mixture of materials, and in the several steps and the relation of each of the same to one or more of the others, as more particularly described herein, the scope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features of our invention, it may be noted at this point that stainless steel is defined as an alloy comprising carbon usually from appreciable amounts up to about 0.25% or more, about to 35% chromium, with or without nickel, and with or without supplemental additions of manganese, silicon, cobalt, copper, molybdenum, tungsten, vanadium, columbium, titanium, sulphur, and the like, for special purposes, and the remainder substantially all iron.

It may also be noted that stainless steel ordinarily is characterized by a dull-grayish appearance. Under certain circumstances, however, the steel may be brightened, even to the extent of polishing, to a brilliant mirror-like surface. There are methods of polishing in the prior art which, for example, involve mechanical steps including abrading, buffing or the like. These steps are often applied to the dull-grayish steel as after pickling and, in being of a mechanical nature, frequently are more tedious and time-consuming than is tolerable for particular needs, gspefiially where depended upon alone to achieve bright nis More recent processes for polishing stainless steel surfaces introduce electrolytic treatment, such as where the metal is immersed and made the anode of an electrolytic bath. Immersion treatment has the advantage of exposing the surface of the work to be polished to the polishing effect regardless of the shape of the surface. The electrolytic processes, however, impose demand for electrical supply, and it often becomes necessary in this respect to install expensive batteries or a motor-generator set for producing current in'the polishing system. These processes frequently are of a kind which expose the stainless steel products to hurtful effects, such as those leading to surface burns, and thus require a considerable amount of care in use if damage to the steel is to be avoided. It follows that the electrolytic processes not only require attention from the standpoint of proper connections in the operating system, but introduce the expense of power consumption over and above the cost of other factors.

An outstanding object of our invention accordingly is the provision of an economical, industrially practical immersion process for polishing stainless steel, wherein the work is efiectively and satisfactorily polished with a min- 2,694,001 Patented Nov. 9, 1954 imum of equipment and with simple control over operat ing conditions.

Referring now more particularly to the practice of our invention, we subject stainless steel to the action of an aqueous solution containing nitric acid, hydrochloric acid, sirmlju'rifqacid, preferably. hydrofluoric acid, and one or more aromatic or aliphatic organic compounds having High molecular Weight soluble therein. This solution Ei'ightens the steel. Among the aromatic compounds of the character indicated which we often employ are the u' nes, antllmquinones, caffeine, creosol, creosotes, pheno henol, naphthalene, pydridjne, and sulph- And, we prefer to employ, our brightening solutions, those of the alkaloid class, illustratively vegeta oids such as uinoidine, chinchgniue and n' co lme, or the animal alkaloids Had By the distillation of animal matter such as hair, hides, hoofs and the like. Commercially, quinoidine has proved to be most practical in our solutions from the viewpoints of expense, life and general efficiency, and accordingly this alkaloid is most preferred. Good results also are had with the aromatic compound sulfurized quinoidine and Wlltgd the aliphatic compound hydroxyethyl thiazolinyl su e.

The various solutions which we provide exert a picklipg action on file stainless-steerand'atsuanrignrentpgetfeet. firirbrightening -is-made possibie thmuglfthe' use of the organic compound or compounds and the particular acids. In certain instances, we provide solutions of the character indicated which have a brightening effect to the extent of polishing.

In the solutions identified herein, the various acids preferably are concentrated to at least about the extent indicated as follows:

Hy6d0rpcll 1loric acid-Specific gravity 1.18 minimum at The exact function of quinoidine or of other organic compound of high molecular weight in giving the brightening effect is not entirely clear to us. It is, further, not known to what extent these materials alter in the presence of the acids. There is a possibility that the alkaloidsor other organic compound in the particular acid environment create a dilference in electrical potential between high and low spots on the stainless steel with eventual leveling of the metal surface to produce the luster. Again, it is possible that the organic material acts as a wetting agent thus facilitating acid reaction on the high spots as compared with low spots on the steel. While the end results of the brightening treatment are quite satisfactory, We do not wish to be bound by any explanation here as to the intricacies of the brightening action.

In the practice of our invention, we find that quite satisfactory results are had, to the extent of pickling and polishing, through the use of a solution containing by volume, approximately 2% to 8% nitric acid, 3% to 30% hydrochloric acid, 2% to 32% hydrofluoric acid, 3% to 18% sulfuric acid. and the remainder substantially all water, together with one or more organic compounds of high molecular weight in total amount of about .5 to 6% by weight of bath. Preferably, however, the bath contains approximately 3% to 5% nitric acid, 3% to 6% hvdrochloric acid. 2% to 12% hydrofluoric acid. 3% to 8% sulphuric acid, and the remainder substantially all water. the percentage figures being by volume, together with aromatic organic compounds, as of the alkaloid class, amounting to about 0.05% to 6% by weight of the bath. Upon subiecting the steel to this solution under proper conditions a polish is readily had ranging in degree from low to high depending upon the specific amounts of the solution constituents present.

A specific polishing solution which we prepare and use for achieving high polish in the practice of our invention, contains approximately 3% to 5% nitric acid, .3% to 6% hydrochloric acid, 2% to 6% hydrofluoric acid, 3% to 5% sulphuric acid, and remainder substantially all water, the percentage figures being by volacid-resistant lining or surface.

range just noted throughout immersion.

time, together with aromatic organic compound having "high molecular weight "in total amount "ranging 'from about 2% to 4% by weight of bath.

A polishing solution which we often provide for achieving a moderate polish, or :more generallya brightening effect, conveniently includes about 2% 'to 8% 'nitric acid, 4% to hydrochloric-acid, 2% to 12% sulphuric acid, and the remainder substantially all water, the percentage figures being by volume, together with aromatic compound of high molecular weight rangmore, the heating for example being with the solution contained in a -vat, tank, or .the like having a suitable Brightening -.is advantageously had by immersing the steel in the heated solution with solution temperature maintained within the The products ilustratively are immersed as a batch in the brightening :solution or, for example, are continuously fed through the solution while supported on a moving rack. Sometimes the products are in the form of stainless-steel wire, =sheet, strip, or the like, which for example is contin- -uously pulled from a coil or roll through the heated 'solution and is re-wound in the brightened condition on a suitable reel as after rinsing in clean water.

The time required for brightening stainless :steel in the particular solutions noted above usually ranges from about three minutes to twenty-five minutes, considering the initial condition of the steel and that the activity of the bath increases with the temperature. About a fifteen minute period of immersion is the optimum time of treatment. In our baths, which are satisfactory for brightening to the extent of polishing, a polish usually is had on stainless steel within the same range of time as indicated, where the bathtemperatures .approximate 160 'F. to 185 F.

In treating cold-drawn stainless steelproducts, we often use solution temperatures toward the lower end of the temperature range of 160 F. to 185 F., and for hotworked products, we usualy employ the higher temperatures of the approximate range.

Our immersion process is useful for brightening stainless steels whether the steels be of straight-chromium grade or of chromium-nickel grade. The chromiumnickel steels are particularly responsive to treatment in our brightening and polishing solutions, though the straight-chromium steels also readily respond to treatment in accordance with our invention. Upon stainless-steelof eithergeneraLgrade. e rightening i r miqkli i qti saranlziqhuiseflleefiteis removing such surface defects from the steel.,as ,sli vers, infidded scale, 'pits, "linfejs,""andthe e, as may for example .be ,caused by teemifigllifiiculfi mmmhgor other processing. Along with the picklingaction, brightnessis'idiieved. Our invention, accordingly, is useful for such purposes as finishing stainless steel inthat it supplies in a unique operation a means for disposing of blemishes and imperfections and, at the same time, produces a bright surface. The process is .useful in such instances as where the stainless steel immersed for treatment has previously been cast, hot-worked or colddrawn, shaved, ground or machined; wherever brightness is desired or where surface imperfections are to be removed.

Sometimes before subjecting the stainless steel to the action of the brightening bath in accordance with our invention, we immerse the steel in an ordinary pickling solution such as one containing suitable amounts of nitric .acid, hydrofluoric acid and the remainder substantially all water to remove surface defects. In this manner, for example, we preliminarily clean the steel to condition the same for our brightening treatment. We subsequent- 1y immerse the steel in any of our brightening solutions containing the aromatic compound orcornpounds for the brightening effect. While our solutions are quite effective for exerting a pickling action on the steel,.the

preliminary cleaning treatment in a relatively cheap pickling solution or the like is sometimes to the interests of enonomy. By immersing a preliminarily cleaned steel product in the brightening solution, demands'upon the latter become relatively light such as to obtain a final pickling action and the brighteniugeffect.

We often use our brightening solutions in any of a "variety of" conventional pickling tanks," such as made of wood, carbon brick or with non-siliceous acid'resisting linings. The solutions readily operate at temperatures which promote brightness without rapid deterioration or loss of the chemicals present. Our solutions are not as potent in concentration as to be objectionably dangerous. In time, of course, the brightening bath suffers a chemical breakdown and loss of efliciency. Most of our baths -thoughare capablebf enduring prolonged .use, and by way of examplesome 20,000 pounds of 18-8 stainless steel wire have been polished in 600 gallons of one of our brightening solutions before deterioration of the latter. We often: temporarily replenish the bath with any one or all of theacids normally in solution. When the amount of iron yielded to the bath reaches too great a quantity however(about 5.5% to 7.5%) the acids usually decompose and the bath is best fully replaced.

In certain instances, we introduce :thio-cyanate or other suitable anti-freeze material or materials to :our solutions, thus to make the latter more satisfactoryfor shipment or toprevent freezing where cold in the bright- ='ening tank.

As illustrative of the practice of.our invention several specific examples of our polishing solutions and :uses of .the same are noted below.

Example I A number of coils of 17% chromium-7% nickel stain- .lesssteel wire in the hot rolled, annealed and pickled condition, measuring l4 inch, ,1 inch and /3 inch in diameter and weighingin total 5966 pounds were treated in a 600-gallon bath containing:

5% concentrated, sulphuric acid by volume.

35 5% concentrated hydrochloric acid by volume.

4% concentrated nitric acidby volume.

4% concentrated hydrofluoric acid by volume.

Balance water, together with quinoidine in the amount of12'% 'by weightof bath.

50 "be unimportant. These polished coils were rinsed in cold water to remove the polishing solution, and thereafter were cold drawn-and shipped for'fabrication to a customer plant. We find the life of this bath to be'par- .ticularly good.

jExamplev II Inthe samebath-as'in Example I, "3280 pounds'of --hot 'rolled, annealed and pickled 17% chromium'stain- -less steel wire, measuring 0.330 inch in diameter, were treated by immersion with prevailing temperature of .about with favorable results. The immersion time in this instance was shortened to 12 minutes.

:ZExnmple III A bath containing the following ingredients was prepared in a suitable polishing tank for immersion treat- .ment of a 5850pound lot of cold-drawn and annealed 18-8 chromium-nickel stainless steel wire.

5% concentrated sulphuric acid by volume.

"5% "concentrated hydrochloric acid by volume.

4% concentrated nitric acid by volume.

4% concentrated hydrofluoric acid .by volume.

'Balance water, together with insoluble distillate of ani- "mal, protein in the amount of 0.15 by weight of bath.

'The immersion treatment lasted for 12 minutes at about 170f F. and this was sufiicient to impart a thoroughly satisfactory polish.

Example I V .Straight chrorr ium :and chromium-nickel stainless steel products,-respect1vely of 17% chromium and l8-8 .chromium-nickel variety were successfully polished in the hot rolled, annealed and pickled condition by immersion in the following polishing solution for about a 15 minute period of treatment.

% concentrated sulphuric acid by volume.

5% concentrated hydrochloric acid by volume.

4% concentrated nitric acid by volume.

4% concentrated hydrofluoric acid by volume.

Balance water, together with creosote in the amount of 5% by volume of bath.

Thus, it will be seen that in this invention there are provided a process and treating solution for improving the surface condition of stainless steel, in which the various objects noted herein together with many thoroughly practical advantages are successfully achieved. It will be seen that the process is simple to practice and that the treating solutions which we provide lend ease and effectiveness in brightening the steel.

As many possible embodiments may be made of our invention and as many possible changes may be made in the embodiments hereinbefore set forth, it will be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

We claim:

1. In polishing a stainless steel product comprising chromium, nickel, and carbon in which the chromium content amounts to about to 35% and remainder substantially iron, the art which comprises, immersing the product to be polished in a bath essentially consisting of by volume about 2% to 8% nitric acid, 4% to hydrochloric acid, 2% to 12% hydrofluoric acid, 3% to 18% sulphuric acid, and the remainder substantially water together with at least one organic compound of the alkaloid class amounting to about 0.05% to 6% by weight of bath for sufiicient time and at such temperature as to achieve a polishing of the product.

2. In polishing a stainless steel product comprising chromium, nickel, and carbon in which the chromium content amounts to about 10% to and remainder substantially iron, the art which comprises, immersing the product in a bath essentially consisting of by volume about 3% to 5% nitric acid, 3% to 6% hydrochloric acid, 2% to 6% hydrofluoric acid, 3% to 8% sulphuric acid, and the remainder substantially water, together with organic compound of the alkaloid class, amounting to about 0.5% to 6% by weight of bath, for such time and at such temperature as to achieve a mirror-like finish.

3. Art according to claim 1 wherein the organic compound of the alkaloid class is quinoidine 4. Art according to claim 1 wherein the organic compound of the alkaloid class is nicotine.

5. Art according to claim 1 wherein the organic compound of the alkaloid class is chinchonine.

6. Art according to claim 1 wherein the organic compound is an animal alkaloid.

7. Art according to claim 1 wherein the organic compound is an aromatic alkaloid.

8. A polishing solution for stainless steel containing chromium, nickel, and carbon wherein the chromium content is about 10% to 35 and remainder substantially iron, said solution essentially consisting of by volume about 2% to 8% nitric acid, 4% to 15% hydrochloric acid, 2% to 12% hydrofluoric acid, 3% to 18% sulphuric acid, and the remainder substantially water, together with at least one compound of the alkaloid class gmtgunting in total to about 0.05% to 6% by weight of 9. A polishing solution for stainless steel containing chromium, nickel, and carbon wherein the chromium content is about 10% to 35% and remainder substantially iron, said solution essentially consisting of by volume about 3% to 5% nitric acid, 3% to 6% hydrochloric acid, 2% to 6% hydrofluoric acid, 3% to 8% sulphuric acid, and the remainder substantially water, togielther with .5 to 6% vegetable alkaloid by weight of bat 10. A polishing solution for stainless steel containing chromium, nickel, and carbon wherein the chromium content is about 10% to 35% and remainder substantially iron, said solution essentially consisting of by volume about 3% to 5% nitric acid, 3% to 6% hydrochloric acid, 2% to 6% hydrofluoric acid, 3% to 8% sulphuric acid, and the remainder substantially water, together with 0.5 to 6% quinoidine by weight of bath.

11. A polishing solution for stainless steel containing chromium, nickel, and carbon wherein the chromium content is about 10% to 35% and remainder substantially iron, said solution essentially consisting of by volume about 5% concentrated sulphuric acid, 5% concentrated hydrochloric acid, 4% concentrated nitric acid, 4% concentrated hydrofluoric acid and the remainder water, together with quinoidine in the amount of 2% by weight of bath.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,742,986 Carson Jan. 7, 1930 2,033,444 Nisizawa Mar. 10, 1936 2,177,751 Sikorski Oct. 31, 1939 2,337,062 Page Dec. 21, 1943 2,446,060 Pray July 27, 1948 2,511,988 Meyers June 20, 1950 OTHER REFERENCES Monypenny-Stainless Iron and Steel, London, Chapman and Hall, Publisher, Cpw. 1931, pages 495 and 493.

Monypenny-Ool. 1) Cpw. 1951, pages -183. 

1. IN POLISHING A STAINLESS STEEL PRODUCT COMPRISING CHROMIUM, NICKEL, AND CARBON IN WHICH THE CHROMIUM CONTENT AMOUNTS TO ABOUT 10% TO 35% AND REMAINDER SUBSTANTIALLY IRON, THE ART COMPRISES, IMMERSING THE PRODUCT TO BE POLISHED IN A BATH ESSENTIALLY CONSISTING OF BY VOLUME ABOUT 2% TO 8% NITRIC ACID, 4% TO 15% HYDROCHLORIC ACID, 2% TO 12% HYDROFULORIC ACID, 4% TO 15% 18% SULPHURIC ACID, AND THE REMAINDER SUBSTANTIALLY WATER TOGETHER WITH AT LEAST ONE ORGANIC COMPOUND OF THE ALKALOID CLASS AMOUNTING TO ABOUT 0.05% TO 6% BY WEIGHT OF BATH FOR SUFFICIENT TIME AND AT SUCH TEMPERATURE AS TO ACHIEVE A POLISHING OF THE PRODUCT. 